1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and more particularly relates to a semiconductor device that stores a defective address in a nonvolatile memory element and a manufacturing method of the semiconductor device.
2. Description of Related Art
While storage capacity of semiconductor memory devices represented by DRAM (Dynamic Random Access Memory) is increasing every year due to the progress of downscaling technique, the reality is that the number of defective memory cells included in a chip is also increasing as chips are further downscaled. Such defective memory cells are usually replaced by redundant memory cells, and thus defective addresses can be relieved.
Fuse elements are used to store defective addresses (see Japanese Patent Application Laid-open Nos. H10-75170 and 2006-147651). Fuse elements in an initial state are electrically conductive and can store a defective address in a nonvolatile manner by laser beam irradiation that disconnects the conduction. Therefore, a desired address can be stored by providing a plurality of such fuse elements and disconnecting a desired one of the fuse elements. That is, the fuse element is a device that stores information in a nonvolatile manner by turning its conductive state to an insulating state.
However, defective bits may also be generated sporadically after an address replacement described above is performed, due to heat stress during packaging or the like. When such defective bits are found after packaging, the packaged item needs to be handled as a defective product, because the address replacement by laser beam irradiation can no longer be performed.
To solve the above problem, there has been proposed a method of providing a relief circuit that can relieve a small number of defective bits found after packaging. In this case, electrically-writable nonvolatile memory elements are used for a circuit that stores defective addresses, instead of fuse elements that require laser beam irradiation. A so-called “anti fuse element” that uses dielectric breakdown of an oxide film has been known as such memory elements (see Japanese Patent Application Laid-open Nos. 2004-227361 and 2007-116045).
Contrary to the fuse element, the antifuse element is a device that stores information by turning its insulating state to a conductive state. A defective address is written in the antifuse element by dielectric breakdown caused by application of a high voltage. Therefore, unlike the fuse element, laser beam irradiation is unnecessary when writing. Accordingly, the defective address can be written at a high speed without requiring a device such as a laser trimmer. Furthermore, its product reliability can be improved because problems such as breakage of passivation films by laser beam irradiation are not caused.
Meanwhile, antifuse elements have a problem of time degradation. That is, if dielectric breakdown is insufficient when changing an antifuse element from an insulating state to a conductive state, resistance of the element rises over time and the conductive state returns to the insulating state. Therefore, there has been recently proposed a technique of using a pair of antifuse elements for storing one-bit information (see Japanese Patent Application Laid-open No. 2007-116045). In this technique, a pair of antifuse elements is connected in parallel such that stored information is not lost even when one of the antifuse elements returns to an insulating state, because a conductive state is maintained by the other antifuse element.
Among antifuse elements, there can be a defective product that cannot be turned to a conductive state even when a high voltage is applied. Such a defective product is detected by performing a test in which, after writing a defective address, reading is performed by a method similar to a normal reading method and then whether a correct defective address has been read is verified.
However, with a pair of antifuse elements having one-bit information stored therein as described in Japanese Patent Application Laid-open No. 2007-116045, when only one of the antifuse elements is defective and the other one is not defective, the test mentioned above cannot detect the pair as a defective product. That is, because correct information is read as long as one of the pair of antifuse elements is in a conductive state, the pair cannot be detected as a defective product even when the other antifuse element is not in a conductive state.
As described above, the object of storing one-bit information in a pair of antifuse elements is to reduce the time degradation risk. To achieve this object, both of the two antifuse elements have to function normally at least at the time of production. Therefore, it is desired that even when only one of the antifuse elements is defective, the pair of antifuse elements can be detected as a defective product in a test after writing.